Conventional nickel-metal hydride storage batteries are composed of a positive electrode containing nickel hydroxide, a negative electrode containing a hydrogen storage alloy, a separator interposed between the positive and negative electrodes, and an electrolyte. In the separator, a non-woven fabric made of polyolefin is used, and in the electrolyte, an aqueous solution of potassium hydroxide is used (Power Sources 12, Research and Development in Non-mechanical Electrical Power Sources, 1989, p 393–410.).
The conventional nickel-metal hydride storage batteries have a problem such that, during the repeated charge/discharge cycles at a high temperature, the hydrogen storage alloy contained in the negative electrode is corroded to produce an oxide or a hydroxide because the alloy reacts with the electrolyte. This reaction consumes the water in the electrolyte. Accordingly, when the alloy is corroded, the amount of the electrolyte within the separator is decreased to increase the internal resistance of the battery, resulting in lower cycle characteristic. Additionally, since the constituent element of the hydrogen storage alloy dissolved in the electrolyte can migrate to the positive electrode through the separator, the self-discharge of the battery is accelerated.
In order to cope with the corrosion of the hydrogen storage alloy, changes in the hydrogen storage alloy composition, surface treatments of the alloy and the like have been examined.
In order to deal with the increase in the internal resistance of a battery, improvements in the separator has been investigated. If the hydrophilicity of the separator is improved, the internal resistance of the battery is unlikely to increase even if the amount of the electrolyte in the battery is decreased. This, however, arises problems such as the requirement of a step of enhancing the hydrophilicity of the separator, and the increase in the production cost of the battery.
With regard to the self-discharge, Japanese Laid-Open Patent Publication No. Hei 5-258767 proposes to include a water absorbent polymer in an electrolyte in order to reduce self-discharge during storage of the battery. This method, however, has a disadvantage that the water absorbent polymer is distributed unevenly between the positive and negative electrodes. The uneven distribution of the water absorbent polymer leads to uneven proceeding in the battery reaction; thus, the cycle life of the battery cannot be expected to improve.
U.S. Pat. No. 5,541,019 proposes to use a polymer electrolyte in a nickel-metal hydride storage battery in order to prevent the leakage of the electrolyte. The polymer electrolyte has low gas permeability. Accordingly, a sealed nickel-metal hydride storage battery containing a polymer electrolyte has a disadvantage that the internal pressure of the battery tend to increase if a gas is generated by decomposition of water due to overcharge.